1. Field of the Invention
The present invention relates generally to methods and apparatus for adjusting hydraulic brake pistons, for example, to compensate for brake friction surfaces wear, and more especially to the reduction of stress concentrations in internal automatic adjusters for such brake pistons.
2. Description of the Related Art
It is common knowledge that vehicle braking action over a period of time causes wear of the brake pads or other friction surfaces resulting in increased clearance between the braking surfaces and requiring a longer stroke of the brake actuator piston to effect braking. At one time this problem was alleviated by periodic manual adjustment of the brake system. Automatic adjustment schemes have largely eliminated the need for such periodic manual adjustments.
For example, in a piston actuated multi-disk brake, the brake is actuated by one or more pistons which extend axially from the brake structure and bear against the brake disks or pads. As the brake wears, a greater amount of piston extension is needed to actuate the brake. An adjuster mechanism allows the piston to maintain a set range of travel during actuation despite the increasing total extension. This is commonly accomplished by the use of a tube and expander adjuster system. A common adjuster mechanism employs an expander which is pulled through a plastically deformable tube, and in the process, adjusting the retracted position of the piston.
In this type system, a tube is forced against an internal expander causing the tube to plastically deform when a certain load is reached. This deformation allows the tube to slide past the expander as long as sufficient load is maintained. Typically, the expander is attached to a pin, which is restrained by the brake structure. The tube is held against the expander by a preloaded spring placed in series between the piston and the tube. When brake pressure is applied the piston travels and the spring is further compressed, causing a small increase in load in the adjuster. The piston must travel through a small running clearance before engaging the brake. After engaging the brake, nearly all of the incremental piston force is delivered to the brake, with a small amount split off to further compress the spring. When the piston travels far enough, a direct load path from the piston to the adjuster occurs causing a significant increase in the adjuster load. When a sufficient portion of the piston load is split off into the adjuster, an adjustment event occurs and the tube begins to deform or pull through. The load needed to deform the tube is known as the pull through load. Any additional piston travel beyond this point will produce a finite amount of adjuster pull through. When brake pressure is removed, the spring forces the piston to retract to a rest position. If any pull through has occurred, then the piston will come to rest in a position slightly extended beyond its previous rest position. This incremental extension corresponds to the increment in brake wear since the last adjustment event or occurrence of pull through.
The adjuster tube is typically a thin walled metal tube with a lubricant applied to its inside surface. The expander often resembles a donut and has an outside diameter slightly larger than the inside diameter of the tube. The expander may be attached to the adjuster pin by being threaded onto the end of the pin or held on by a separate nut. High stresses in the threaded portion of the pin typically limit the service life of the pin and the reliability of the adjuster mechanism.
U.S. Pat. No. 4,751,985 illustrates a system where the brake piston adjuster mechanism includes a radially outwardly expandable or deformable adjuster tube which is attached to the brake piston and engaged by a fixed deforming member comprising a cantilevered adjuster pin having a threaded free end receiving thereon a nut and tube expander. As braking surfaces wear, the piston stroke during brake actuation increases and becomes sufficient to move the adjuster tube axially along the tube expander deforming the tube and establishing a new retracted or rest position for the piston. This system accomplished the desired adjustment, but had excessive stress regions in the area of the pin threaded end.
U.S. Pat. No. 5,154,262 shows a technique for alleviating the stress regions while retaining the deforming member structure of a pin having a threaded end receiving a nut and tube expander. Here, the nut biases the tube expander against a chamfered pin shoulder. The tube expander has at least one inner diameter chamfer which engages the chamfered pin shoulder in order to impose preload forces upon the chamfered pin shoulder and reduce stresses at a smaller diameter portion of the pin. This patent illustrates several variations on the nut/expander configuration. An annular expander could be held in place by a castellated nut which receives a locking wire or pin extending through an opening in the threaded end of the pin. Alternatively, the tube expander and nut could comprise a one-piece expander nut having an enlarged radially extending end portion and the nut, when advanced along the threads of the pin, causes a chamfer of the end portion to advance along the chamfered pin shoulder and causes the end portion to deform elastically, improving the preload retention of the connection. In some implementations of these patented systems, the adjuster pin fixed end was square so as to not rotate about its axis when the nut was unthreaded. Brake pad replacement requires replacement of the adjuster tube. While this patented arrangement allowed removal of the adjuster tube without the need for removing the entire brake bushing portion of the housing, the square end was also susceptible to high stress region failure. When split ball adjuster pin failures have occurred, these failures frequently occurred near the small radius in the pin groove. In other implementations, the fixed end was conical and not susceptible to such failures, however, the conical fixed end configuration, which was provided with an allen wrench socket, required removal of the entire piston bushing from the remaining portion of the brake piston housing to gain access to the conical end to effect removal of the nut and replacement of the adjuster tube.
While these prior patented arrangements generally achieved their desired goals, some shortcomings remained. Elimination of the square fixed end stress regions required the undesirable removal of the brake piston bushing during maintenance and adjustment tube replacement. The threaded free end of the adjuster pin still experienced undesirably high stress regions near the base of the threads and where the castellated nut locking wire or cotter pin hole passed transversely through the pin. Due to the limited number of discrete angular positions for the castellated nut, appropriate torquing of the nut to establish the initial location and preload of the tube expander within the adjuster tube was difficult. U.S. Pat. No. 6,234,279 B1 solved many of these difficulties by employing an expandable tube or sleeve which surrounds a cantilevered pin having a split ball or split expander ring near a free end thereof. The split ball adjuster works in principle like any other tube and expander adjuster. It differs in how the expander is attached to the pin. The donut shaped expander is split into two essentially semicircular halves. This allows the expander to be placed into an annular groove in the pin and is then held there by the engaged adjuster tube. Consequently, the expander is attached to the pin without the use of a threaded connection. The benefits of a split ball adjuster are derived from the elimination of the threaded connection. These benefits include the potential for lower stresses in the pin and easier adjuster assembly. In this patented arrangement, the split ring halves are held in position within an annular groove of generally rectangular cross-sectional shape by the deformable adjuster tube. Upon excessive piston travel, the expander moves within the tube to establish a new retracted piston position. Depression of the piston to its initial position allows removal of the split ring and expandable tube allowing ready substitution of a new tube during brake maintenance.
This patented design employs an annular groove having square sides and very small fillets. The revolved cross section of the expander also has square sides and small rounds where it interfaces with the pin. A square side of the expander bears against a square side of the pin groove to transfer adjuster loads into the pin. The attendant small fillets in the groove cause locally high stresses in the pin.
It is desirable to still further reduce the likelihood of adjuster failure in braking systems of this type. This may be achieved by optimizing the shape of the expander/pin interface.
The present invention provides solutions to the above concerns and improves on the existing adjuster mechanisms by reducing the stresses in the key components. In particular, the adjuster pin/expander geometry is optimized to reduce the contact stresses and reduce the stress concentration in the pin. The groove in the pin of the split ball adjuster is no longer square sided, but has a curved cross section. The expander cross section similarly is not square, but is curved to ideally mate with the curved cross section of the annular groove in the pin. This curved groove shape results in significantly lower stresses in the adjuster pin as compared to either a threaded or square groove pin.
This pin/expander uses a relatively large radius on the respective contacting surfaces to minimize the contact stresses. While this presents an angled bearing surface, the stiffness of the adjuster tube, coupled with the direction of the surface normals where contact is made between the pin and expander relative to that of the expander and tube retains the expander in the pin groove without need of an additional component.
The invention comprises, in one form thereof, a piston adjuster mechanism having a deformable member disposed within and connected with a piston and a deforming member received within and engaging the deformable member. The deforming member takes the form of an elongated generally cylindrical cantilevered pin having an annular groove near its free end and a split annular expander ring received within the annular groove. The expander has a substantially identical pair of ring halves which, when joined, provide a smooth annular outer surface for engaging the deformable member in a first annular region, and a smooth annular inner surface for engaging the groove in a second annular region. In one preferred form, the areas where inner and outer contact occurs on the expander are constructed with constant radius curves. It is important that there be no perturbations to the smooth, continuous contour where contact takes place. The area of active contact is very small. Outside of the active contact area the groove can be a composite of several shapes but should be generally concave and the expander convex.
Either one or both of the smooth annular surfaces may be xe2x80x9cflat-freexe2x80x9d, that is, have, in cross section, a finite radius of curvature throughout the region of contact. Either one or both of the smooth annular surfaces may be free of sharp corners, that is, have cross-sectional curve configurations which are continuous and have continuous first derivatives. Again, one or more of the contact surfaces may have a radius of curvature which begins at one edge with a rather small radius of curvature, increases to a greater radius of curvature and then decreases to a small radius of curvature near the opposite end of the surface. The increases and decreases may be monotonic. These curves may, for example, comprise tangentially joined conic section curves. Many other curves may generate the smooth annular surfaces. Either one or both of the contact regions may include more than one of these features.
An advantage of the present invention is that the easy removal and replacement of the adjuster tube is retained while reducing stress.